Add cpp tag to some cpp error tests (#3033)

tests/errors/cimport_attributes.pyx

 # mode: error
+# tag: cpp
 
 
 cimport libcpp
@@ -24,8 +25,8 @@ print my_map_with_shadow.python_attribute   # OK (if such a module existed at ru
 
 
 _ERRORS = u"""
-5:12: cimported module has no attribute 'no_such_attribute'
-8:16: cimported module has no attribute 'no_such_attribute'
-11:12: cimported module has no attribute 'no_such_attribute'
-14:15: cimported module has no attribute 'no_such_attribute'
+6:12: cimported module has no attribute 'no_such_attribute'
+9:16: cimported module has no attribute 'no_such_attribute'
+12:12: cimported module has no attribute 'no_such_attribute'
+15:15: cimported module has no attribute 'no_such_attribute'
 """

tests/errors/cpp_object_template.pyx

 # mode: error
+# tag: cpp
 
 from libcpp.vector cimport vector
 
@@ -12,6 +13,6 @@ def main():
     va.push_back(A())
 
 _ERRORS = u"""
-9:16: Python object type 'Python object' cannot be used as a template argument
-11:16: Python object type 'A' cannot be used as a template argument
+10:16: Python object type 'Python object' cannot be used as a template argument
+12:16: Python object type 'A' cannot be used as a template argument
 """